update Dockerfile

backend/Dockerfile

@@ -2,10 +2,10 @@
 # Stage 1: 依赖安装 + 构建
 FROM node:22-alpine AS builder
 
-ENV PNPM_APPROVE_BUILDS=*
-
 WORKDIR /app
 
+ENV PNPM_APPROVE_BUILDS=*
+
 # 先装依赖（缓存最大化）
 COPY package.json pnpm-lock.yaml ./
 RUN corepack enable && pnpm install --frozen-lockfile